The long term objective of this project is to define cellular growth- regulating pathways that are abrogated by the Adenovirus E1A oncogene products ability to bind a 300-kilodalton host cell protein, p300. p300 is a stable nuclear phosphoprotein that is expressed in several types of tissues and in a variety of mammalian species. E1A genetics implicate p300 in E1A's ability to induce a GO/G1 exit, stimulate DNA synthesis, inhibit cell differentiation and both transcriptionally activate and repress gene expression. Analysis of the p300 gene sequence reveals that p300 has a high degree of amino acid homology and structural features conserved by transcriptional coactivators suggests that it may function as such. Transcriptional coactivators are recruited by activated DNA-binding transcription factors and together they stimulate transcription by interacting with the basal transcription machinery. Significantly, while it is not known if p300 functions as a transcriptional coactivator, a protein that is closely related to p3OO, does: An activated phosphorylated form of CREB (CAMP response element-binding protein) recruits CBP (CREB- binding protein), a p3OO-like protein, to the CRE (CAMP response element) and transcription is stimulated. Since p3OO is found in TBP (TATA-box binding protein)-specific immunocomplexes, at least the aspect that a transcriptional coactivator would then interact with the basal transcriptional machinery is established with p300. To further investigate the cellular p3OO function, I have developed a series of monoclonal antibodies that are p300-specific. A subset of these antibodies identify several putative p300-associated proteins, some of which are phosphorylated. Two of these phosphoproteins, p64 and p59, are also found in TBP-specific immunocomplexes. The identity or functions of these proteins are not known, nor is the specific protein/protein interaction in these cellular complexes. As an initial investigation, I propose to characterize, identify and clone p64 and p59, determine the function of p3OO, p64, and p59 in cell-growth regulation, TBP-specific transcription complexes and test for E1A-dependent abrogation of these functions.